Colloidal microgels have numerous attractive properties such as defined morphology, high porosity and adjustable dimensions that can respond to changes in temperature, pH and solvent quality, and the ability to act as carriers for drugs, biomolecules, synthetic polymers or inorganic nanocrystals through fluid media. As a consequence, these materials are becoming increasingly important for their potential applications in drug and gene delivery, catalysis, sensing, fabrication of photonic crystals, and separation and purification technologies, see a) R. Pelton, Adv. Colloid Interface Sci. 2000, 85, 1; b) S. Nayak, L. A. Lyon, Angew. Chem. Int. Ed. 2005, 44, 7686; c) N. A. Peppas, J. Z. Hilt, A. Khademhosseini, R. Langer, Adv. Mater. 2006, 18, 1345; d) M. Das, H. Zhang, E. Kumacheva, Annu. Rev. Mater. Res. 2006, 36, 117; and J. Zhang, S. Xu, E. Kumacheva, J. Am. Chem. Soc. 2004, 126, 7908.
In such systems, the microgel particles fulfill several important functions, namely a) stabilization and transport of the loaded material in the medium, b) potential controlled release of the load in response to external stimuli, and c) easy recovery by separation from the continuous phase.
Two rather distinct approaches have been taken for loading different substances into microgel particles. The first utilizes the microgel as a template for in-situ preparation of nano-scale materials such as inorganic nanoparticles (NPs). In this case, the nanoparticles are trapped in the microgel interior by hydrophobic forces, hydrogen bonding, or electrostatic interactions. This approach has been realized for both aqueous microgels2 and microgels dispersed in organic solvents, see M. Antonietti, F. Grohn, J. Hartmann, L. Bronstein, Angew. Chem. Int. Ed. 1997, 36, 2080; and A. Biffis, N. Orlandi, B. Corain, Adv. Mater. 2003, 15, 1551. The attractive features of this approach are the effective control of the nanoparticle dimensions within the microgel, and flexibility in control of the nanoparticle loading.
The second approach involves filling the microgel by diffusion of pre-formed nanoparticles into the microgel, accompanied by trapping due to the electrostatic interactions or hydrogen bonding with polymer chains, see I. Gorelikov, L. M. Field, E. Kumacheva, J. Am. Chem. Soc. 2004, 126, 15938; M. Kuang, D. Yang, H. Bao, M. Gao, H. Mohwald, M. Jiang, Adv. Mater. 2005, 17, 267; and Y. Gong, M. Gao, D. Wang, H. Mohwald, Chem. Mater. 2005, 17, 2648. This technique offers some important advantages in terms of the simplicity of the process and independent adjustment of the nanoparticle properties. This approach, however, has been employed primarily in aqueous media and has limited utility for incorporating inorganic nanocrystals synthesized in organic solutions. In both of these approaches, the microgel network serves not only as a container for transporting the nanoparticles, but as a functional unit that can be attached to substrates or respond to stimuli like changes in temperature or pH. By using the two methods described above, a variety of composite microgel particles have been prepared, containing NPs of conducting polymers, (see a) J. Mrkic and B. R. Saunders, J. Colloid and Interface Sci. 2000, 222, 75; b) A. Pich, Y. Lu, H. P. Adler, T. Schmidt and K. Arndt, Polymer 2002, 43, 5723; c) A. Pich, Y. Lu, V. Boyko, Arndt, K.-F. and Adler, H.-J. P., Polymer 2003, 44, 7651; d) Pich A, Lu Y, Boyko V, Richter S, K. Arndt and H. P. Adler, Polymer 2004, 45, 1079; e) E. Lopez-Cabarcos, D. Mecerreyes, B. Sierra-Martin, M. S. Romero-Cano, P. Strunz and A. Fernandez-Barbero, Phys. Chem. Chem. Phys. 2004, 6, 1396; d) J. Rubio Retama, E. Lopez Cabarcos, D. Mecerreyes and B. Lopez-Ruiz, Biosens. Bioelectron. 2004, 20, 1111); noble metals, (see a) G. Sharma and M. Ballauff, Macromol. Rapid Comm. 2004, 25, 547; b) Y. Mei, G. Sharma, Y. Lu, M. Ballauff, M. Drechsler, T. Irrgang and R. Kempe, Langmuir 2005, 21, 12229; c) Y. Lu, Y. Mei, M. Drechsler and M. Ballauff, Angew. Chem. Int. Ed. 2006, 45, 813; d) D. Suzuki and H. Kawaguchi, Langmuir 2005, 21, 12016; e) J. Zhang, S. Xu and E. Kumacheva, Adv. Mater. 2005, 17, 2336; f) A. Pich, A. Karak, Y. Lu, A. Ghosh and H. J. P. Adler, Macromol. Rapid Comm. 2006, 27, 344; g) A. Biffis, N. Orlandi and B. Corain, Adv. Mater 2003, 15, 1551), metal oxides, (see a) M. Gao, X. Peng and J. Shen, Thin Solid Films 1994, 248, 106; b) C. Menager, O. Sandre, J. Mangili and V. Cabuil, Polymer 2004, 45, 2475); metal sulfides, (see a) C. Bai, Y. Fang, Y. Zhang and B. Chen, Langmuir 2004, 20, 263; b) A. Pich, J. Hain, Y. Lu, V. Boyko, Y. Prots and H. Adler, Macromolecules 2005, 38, 6610); and biominerals, (see a) N. Nassif, N. Gehrke, N. Pinna, N. Shirshova, K. Tauer, M. Antonietti and H. Cölfen, Angew. Chem. 2005, 117, 6158; b) G. Zhang, D. Wang, Z. Gu, J. Hartmann and H. Möhwald, Chem. Mater. 2005, 17, 5268; c) G. Zhang, D. Wang, Z. Gu and H. Möhwald, Langmuir 2005, 21, 9143).
In most cases, the composite microgels preserve the colloidal stability and maintain the stimuli responsiveness of the pure microgels. At the same time, the NPs carried by the composite exhibit the typical physical and chemical properties of nanomaterials themselves.
The methods for the preparation of composite microgels described above require the adjustment of the microgel and nanoparticles (or their syntheses) to the nature of the medium, whether water or an organic solvent. The medium in which the composite microgels are formed using these strategies is normally the only medium in which they are stable and can be employed. This limitation can be overcome by the consideration of one important property of the microgel itself, which has not been exploited for microgel-NP composites: the ability of many kinds of microgels to form stable colloidal solutions in solvents of very different polarity.
Some authors have noted the nearly universal ability of microgels to form emulsions (see a) S. Fujii, E. S. Read, B. P. Binks and S. P. Armes, Adv. Mater. 2005, 17:1014; b) T. Ngai, S. H. Behrens and H. Auweter, Chem. Commun. 2005, 3, 331; c) T. Ngai, S. H. Behrens and H. Auweter, Macromolecules 2006, 39, 8171; d) S. Fujii, S. P. Armes, B. P. Binks and R. Murakami, Langmuir 2006, 22, 6818; e) A. Y. C. Koh and B. R. Saunders, Langmuir 2005, 21, 6734), or colloidal solutions in mixed solvents (see H. M. Crowther and B. Vincent, Coll. Polym. Sci. 1998, 276, 46).
Many microgel compositions are soluble not only in water but also in organic solvents. Nevertheless, little attention has been paid, however, to the possibility of transferring microgels from water to organic solvents or from organic solvents to aqueous media. In the present invention the inventors show that by selecting solvents that are both miscible with water and also good solvents for the microgel network,
The inventors also contemplate that if an aqueous solution of the microgel is stirred with an organic solvent such as dichloromethane (DCM), dichloroethane, ethyl acetate, or anisol, which is not miscible with water, the microgel can be induced to transfer from the aqueous medium to the organic phase. One can transfer the microgel from its natural aqueous environment to an organic phase by a solvent exchange process. If this process is reversible and if the microgels retain their stability upon transfer, new designs of composite microgels and their applications become possible.